Books and books have been written on this topic. The classic is Brunt's Italian Manpower, which puts the population of Italy (p.124) at 7.5 million, compared to about 4 million Roman citizens. This is earlier than the period you're asking for; Brunt studies the period between the Second Punic War and the death of Augustus. As for the question about Gaul, I'm not sure any systematic study has been done on population there, but Brunt despaired of estimating the number of Roman citizens living abroad.

More recently Lo Cascio (JRS 84) has estimated the population of first century Italy as high as 12 million. Pfeilshifter (Roman by Integration) has interpreted this number as the carrying capacity of Italy at the time, and the reason for the first century land crisis, a theory which while rejecting all the ancient sources dovetails neatly with Rosenstein's argument that there was no massive depopulation of Italy in the first century.

The difference comes down to how you understand the Augustan census numbers. Brunt assumed the census counted all free citizens, Lo Cascio that they only counted men of military age. I like Lo Cascio's position, but I'm very nervous about throwing out the ancient reports about the land crisis. When I figure it out you can read about it in my book.

EDIT: Oh, I just realized you're asking about "Romanization" not population. That also has tons of ink spilled over it. Crawford in the Cambridge Ancient History says Romanization hadn't penetrated very far into rural Italy at all. Revell (Roman Imperialism and Local Identities) argued that "Romanization" was a local phenomenon practiced in different ways in different places. That's a bullshit argument in my opinion, but she got published so it's worth something. Other scholars following Syme (Cornell I think is the most well known active scholar with this opinion) think assimilation had advanced quite far by the first century.